Chapter 2 USING THE SWITCHABLE QUALITY OF A LIQUID CRYSTAL SOLVENT TO MEDIATE SEGREGATION BETWEEN COIL AND LIQUID CRYSTALLINE HOMOPOLYMERS

Size: px

Start display at page:

Download "Chapter 2 USING THE SWITCHABLE QUALITY OF A LIQUID CRYSTAL SOLVENT TO MEDIATE SEGREGATION BETWEEN COIL AND LIQUID CRYSTALLINE HOMOPOLYMERS"

Maryann O’Connor’
5 years ago
Views:

1 22 Chpter 2 USING THE SWITCHABLE QUALITY OF A LIQUID CRYSTAL SOLVENT TO MEDIATE SEGREGATION BETWEEN COIL AND LIQUID CRYSTALLINE HOMOPOLYMERS Chpter Introduction Experimentl Results Discussion Conclusions Tles Figures References...34 Reproduced in prt with permission from Scruggs, Kornfield, nd Ll, Mcromolecules 2006, 39, Copyright 2006 Americn Chemicl Society. 2.1 Introduction The thermodynmics governing phse ehvior in polymer solutions is strikingly different from tht of regulr, smll-molecule solutions. The entropy of mixing tht fvors misciility etween smll-molecule species is sustntilly reduced y the covlent connectivity of monomers in polymer chin. The usully unfvorle enthlpy of mixing two species is, therefore, more difficult to overcome when one of them is polymer. The competition etween entropy nd enthlpy is summrized y the Flory-Huggins eqution [1]

2 23 which expresses the free energy of mixing, F mix, s function of the polymer s volume frction, φ, nd degree of polymeriztion, N, s well s n interction prmeter, χ, tht quntifies the enthlpy of mixing: F mix φ = lnφ + ( 1 φ) ln( 1 φ) + χφ( 1 φ), (2.1) kt N where k is Boltzmnn s constnt nd T is the temperture. The Flory interction prmeter is usully positive nd hs een found empiriclly to depend on temperture ccording to where A nd B re constnts. [1] B χ ( T ) = A +, (2.2) T Smll-molecule nemtic liquid crystls (LCs) re prticulrly interesting s solvents for polymers ecuse they cn undergo first-order trnsition etween ordered (nemtic) nd disordered (isotropic) fluid phses, nd the χ prmeter chnges discontinuously t the nemtic-isotropic trnsition temperture (T NI ). Below T NI the entropic penlty of dissolving rndom-coil polymer in ordered solvent is lumped into the χ prmeter, which grows with the solvent s degree of orienttionl order, ut ove T NI this penlty is lost nd the LC ehves like conventionl solvent. [2] If, however, the polymer lso hs liquid crystlline chrcter, χ my switch from eing positive to negtive when the solvent ecomes isotropic. In either cse, the misciility of polymer with n LC solvent my chnge drsticlly in the smll temperture window round T NI. Rich thermodynmics result from dissolving coil polymer nd side-group liquid crystl polymer (SGLCP) together in n LC solvent. The ternry phse ehvior of such polymer-polymer-solvent system is determined y the entropy of mixing lnced with polymer-solvent interctions (chrcterized y χ AS nd χ BS ) s well s polymer-polymer interctions (chrcterized y χ AB ). [3-7] In nemtic solvent, ll three χ prmeters chnge discontinuously t T NI nd the solution s phse ehvior is ltered ccordingly. In this chpter, the inry phse ehvior of rndom-coil polymer (polystyrene, PS) in n LC solvent (4-pentyl-4 -cynoiphenyl, 5CB) is compred to solutions of n SGLCP in

3 24 5CB in order to correlte polymer properties with chnges in polymer misciility tht tke plce t T NI. These results elucidte the physics ehind the rich phse ehvior of ternry solutions of PS, SGLCP, nd 5CB tht hs consequences for solutions of coil- SGLCP lock copolymers discussed in lter chpters. 2.2 Experimentl Mterils A side-group liquid crystl homopolymer, 490HSiCB4, ws synthesized nd chrcterized ccording to the methods descried in Appendix A (Figure 2.1). A 63 kg/mol 1,2- polyutdiene (PB) prepolymer ws synthesized vi living nionic polymeriztion y Steven Smith of Proctor nd Gmle, Inc. (see Appendix A for chrcteriztion). After ttching the mesogenic side-groups, the polymer s moleculr weight (M n ) is 489 kg/mol nd 97 mol % of the monomers hve SiCB4 mesogens ttched. Of the remining 4 mol %, 1 mol % of the monomers re residul 1,2-utdiene nd 3 mol % re unrective 1,4- utdiene. The polydispersity (PDI = M w / M n ) is 1.48 (Tle 2.1). Monodisperse polystyrene (PS) homopolymer with M n = 44 kg/mol ws purchsed nd used s received from Aldrich. The nemtic LC solvent 4-pentyl-4 -cynoiphenyl (5CB) ws used s received from TCI Americ Methods Ternry mixtures of 490HSiCB4 homopolymer nd PS homopolymer in 5CB were mde y comining the three components in controlled quntities for totl mss of pproximtely 50 mg then dissolving them together in ~100 µl of tetrhydrofurn (THF). Smples were mixed for t lest n hour to ensure complete dissolution of ll three components. A smll drop of the THF solution ws plced in the shllow well of n indented microscope slide nd the THF ws evported wy t elevted temperture (~100 o C). Slides were exmined using Zeiss Universl opticl microscope equipped with Mettler FP82 hot stge nd removle polrizers. Ech smple ws first heted from room temperture t rte of 2 o C/min to oserve the nemtic to isotropic phse trnsition; the temperture t which the colorful, irefringent texture viewed etween

4 25 crossed polrizers (Figure 2.2) disppered ws recorded s the isotropiztion point (T NI ). Next, the polrizers were removed nd the smple ws heted further until droplets chrcteristic of two-phse, isotropic/isotropic coexistence (Figure 2.2) were no longer oserved (Figure 2.2c). Then, the smple ws cooled t rte of 10 o C/min nd the temperture t which droplets reppered ws recorded s nominl upper criticl solution temperture (UCST). This mesurement ws repeted five times, exmining different re on the slide ech time, nd the results verged. To set ound on the sucooling required for oservle drops to form on cooling, severl smples were rised to 5 C ove the nominl UCST. Consistently, single phse formed over time. Therefore, the true UCST lies within 5 C of the nominl one. Since the chnges in UCST with solution composition were very lrge, this uncertinty did not ffect the conclusions of this study. 2.3 Results Binry solutions of 490HSiCB4 with 5CB were oserved to e single phse oth ove nd elow T NI (Figure 2.3). Binry solutions of PS with 5CB exhiit smll iphsic window in the isotropic phse of 7.4 o C or less (Figure 2.3) nd re single-phse ove n upper criticl solution temperture (UCST). This result is consistent with previously estlished phse digrms of PS with 5CB, [8, 9] which found the two to e miscile ove 40 o C, provided the concentrtion of PS is less thn 50 wt %. Ternry mixtures of PS, 490HSiCB4, nd 5CB were oserved to phse seprte elow T NI into coexisting nemtic nd isotropic phses t ll compositions tested (up to 20 wt %). The mesured isotropiztion points in ternry mixtures (i.e., trnsition from N + I to I + I iphse or N + I to I) were found to e within 3.5 o C of the ulk T NI of 5CB (35 o C), indicting tht the nemtic phse contins little or no isotropic diluent nd prtitioning of the polymers into n SGLCP-rich, nemtic phse nd PS-rich, isotropic phse must therefore e nerly complete. Aove T NI wide misciility gps were oserved up to the UCST.

5 26 The UCST of ternry mixtures of PS, 490HSiCB4, nd isotropic 5CB ws found to e highly sensitive to the concentrtions of the two polymers nd n effective soluility limit ws quickly reched with incresing concentrtions (Figure 2.4). For exmple, holding the concentrtion of 490HSiCB4 fixed t 4.5 wt %, the ddition of 0.5 wt % PS opens up 14 o C misciility gp (UCST = 49 o C) which expnds to 97 o C (UCST = 132 o C) when the concentrtion of PS is 6 wt %. When the overll concentrtion of polymer, PS nd 490HSiCB4 comined, exceeds pproximtely 10 wt %, the UCST cnnot e reched efore the smple thermlly degrdes, representing n effective soluility limit. This steep increse in UCST with incresing polymer concentrtion is surprising ecuse ech of the two polymers dissolves in ll proportions tested in isotropic 5CB lone. 2.4 Discussion Binry nd ternry mixtures of 490HSiCB4 nd PS dissolved in 5CB show tht nemtic 5CB is strongly selective solvent for the SGLCP. As previously reported y Hori et l., [9] PS is insolule in nemtic 5CB; mixtures phse seprte into n isotropic, PS-rich phse nd nemtic phse nerly devoid of PS ltogether. On the other hnd, 490HSiCB4 is solule in nemtic 5CB t ll concentrtions tested (up to 20 wt %). The liquid crystlline order of the nemtic solvent imposes lrge entropic penlty to solvtion of rndom coil polymer (e.g. PS), [9-11] ut the chemiclly similr side-groups nd liquid crystlline nture of the SGLCP fcilitte misciility with the nemtic solvent. [2] The oservtion of lrge misciility gps in ternry mixtures of PS, 490HSiCB4, nd 5CB ws surprising ecuse single-phse solutions re esily chieved in inry mixtures of 5CB with either polymer lone. One reson for the polymers poor soluility in ternry solutions is tht unfvorle interctions etween the polymers themselves ( lrge contriution of χ AB ) increse the free energy of mixing. [7] However, this effect is wek in dilute solutions where polymer-polymer interctions re effectively screened y solvent. Smll-ngle neutron scttering experiments presented in Chpter 5 demonstrte tht 5 wt % solutions of similr SGLCPs (310HSiCB4 nd 780HSiCB4) re in the semidilute regime, ut when the concentrtion of 490HSiCB4 is sustntilly lower the polymer chins should

6 27 e dilute nd non-intercting. The overlp concentrtion of PS is clculted [12] to e pproximtely 7 wt % t 35 C in cyclohexne [13] which represents lower ound on the dilute regime since 5CB is not s good solvent for PS s cyclohexne. A portion of the mesured phse digrm is, therefore, well within regime where the free energy contriution of inter-polymer interction (χ AB ) should e smll. Besides polymer-polymer interctions, polymer-solvent interctions lso contriute to the system s thermodynmics, s descried y the Ptterson [6] -Prusnitz [3] tretment of the Scott [4] -Tomp [5] theory for ternry polymer-polymer solutions. Ptterson nd Prusnitz found tht ny differentil preference of the solvent for one polymer over the other (χ AS χ BS ), even if tht preference is smll, cn induce phse seprtion. Furthermore, the effect is excerted when the moleculr weight difference etween the polymers is lrge. The symmetric solvent effect is, therefore, predicted to ply sustntil role in the thermodynmics governing phse seprtion of solutions of PS nd 490HSiCB4 in 5CB: the chemicl structure of the two polymers is sufficiently different to cuse n pprecile difference in polymer-solvent interctions etween the two, nd the moleculr weight rtio of PS to HSiCB4 is pproximtely 1:10. Isotropic 5CB is, therefore, not neutrl solvent (χ AS = χ BS ), ut is etter clssified s slightly selective ( χ AS - χ BS << χ AS χ BS ). Compred to conventionl solvents, 5CB is unique in its ility to undergo discontinuous, thermlly-induced chnge in its misciility with polymers. Solvent qulity is typiclly monotonic function of temperture, [1] ut the first-order nemtic-isotropic phse trnsition results in n rupt chnge so tht incresing temperture less thn 1 C cn cuse n initilly insolule polymer to ecome miscile with 5CB. This phenomenon is termed switchle solvent qulity. 2.5 Conclusions The switchle solvent qulity of n LC solvent results in rich ternry solution thermodynmics. Unfvorle polymer-polymer interctions work together with strong solvent selectivity to drive mcroscopic phse seprtion in the nemtic phse. In the

7 28 isotropic phse, fvorle interctions etween the solvent nd oth polymers re lnced ginst polymer-polymer incomptiility nd the symmetric solvent effect, resulting in phse ehvior tht is strikingly sensitive to chnges in the solution s composition. Modultion etween the two regimes is chieved with temperture chnges of less thn 1 C. The unique phse ehvior of homopolymer solutions in n LC solvent suggests tht the driving force for self-ssemly of coil-sglcp lock copolymer could lso e modulted y the solvent s trnsition etween the nemtic nd isotropic phses. Below T NI the solvent s orienttionl order would drive the lock copolymer to self-ssemle into structure tht segregtes the coil-lock from the LC host. Aove T NI this driving force would e lost, ut self-ssemly could still occur s result of the sme thermodynmic interctions tht drive phse seprtion in the isotropic homopolymer solutions. This topic is explored in detil in Chpter 3.

8 Tles Tle 2.1 Moleculr weight, conversion, nd polydispersity of the side-group liquid crystl homopolymer. Detils of chrcteriztion re presented in Appendix A. Nme M n [kg/mol] Mole Frction 1,2 PB Mole Frction 1,4 PB Mole Frction LC PDI 490HSiCB PDI = Polydispersity Index (M w /M n )

9 Figures 490HSiCB4 PS 5CB Si O Si C N O C N Figure 2.1 Chemicl structures of side-group liquid crystl polymer (490HSiCB4), polystyrene (PS), nd nemtic liquid crystl solvent (5CB). The SGLCP s nme is derived from its moleculr weight (489 kg/mol), the letter H to indicte homopolymer, nd SiCB4 to indicte end-on ttchment of the mesogens. In ddition to monomers hving n ttched mesogen, the polymer lso contins ~ 1 mol % ech of residul 1,2- nd 1,4-utdiene monomers. Its properties re summrized in Tle 2.1 nd the detils of its synthesis nd chrcteriztion re presented in Appendix A.

10 Figure 2.2 Opticl microgrphs typicl of ternry lends of PS, 490HSiCB4, nd 5CB t tempertures () elow T NI, () etween T NI nd the UCST, nd (c) ove UCST. The smple is imged etween crossed polrizers in the nemtic phse (T < T NI ). These prticulr microgrphs re of 2.25 wt % PS nd 4.57 wt % 490HSiCB4 in 5CB t () 34 C, () 70 C, nd (c) 90 C. 31

11 32 () 50 () T ( C) ( C) T ( C) I N T NI HSiCB4 / 5CB φ (wt. frc.) I I+I N+I φ (wt. frc.) UCST T NI PS / 5CB Figure 2.3 Prtil inry phse digrms of () 490HSiCB4 homopolymer in 5CB nd () polystyrene homopolymer with 5CB. Dshed lines re drwn to guide the eye towrd plusile phse oundries. The letters N nd I indicte single nemtic or isotropic phse, respectively. I+I indictes two coexisting isotropic phses nd N+I indictes coexisting nemtic nd isotropic phses.

12 UCST ( o C) c d e f g h i j k l T NI >140 [HSiCB4] (wt. frc.) l d e d [5CB] (wt. frc.) d j j k k [PS] (wt. frc.) h i l l g Cnnot Access Single Phse l 33 Figure 2.4 Prtil ternry phse digrm of PS (44 kg/mol), 490HSiCB4, nd isotropic 5CB determined from opticl microscopy. The shding nd letter of ech point expresses the upper criticl solution temperture (UCST) t which single-phse solution is otined. A UCST equl to T NI mens the solution ecme single-phse immeditely upon trnsitioning to the isotropic phse. A UCST > 140 C mens single phse ws inccessile ecuse the mixture egn to decompose. The dshed line is drwn to guide the eye to the region where single-phse solution cnnot e reched.

13 References [1] Ruinstein, M.; Coly, R. H. Polymer Physics, 1 st ed; Oxford University Press: New York, [2] Brochrd, F. Solutions of flexile polymers in nemtic liquid. C. R. Acd. Sc. Pris 1979, 289, [3] Hsu, C. C.; Prusnitz, J. M. Thermodynmics of Polymer Comptiility in Ternry Systems. Mcromolecules 1974, 7, [4] Scott, R. L. The Thermodynmics of High Polymer Solutions. V. Phse Equiliri in the Ternry System: Polymer 1- Polymer 2- Solvent. J. Chem. Phys. 1949, 17, [5] Tomp, H. Phse Reltionships in Polymer Solutions. Trns. Frdy Soc. 1949, 45, [6] Zemn, L.; Ptterson, D. Effect of the Solvent on Polymer Incomptiility in Solution. Mcromolecules 1972, 5, [7] Olisi, O.; Roeson, L. M.; Shw, M. T. Polymer-Polymer Misciility, Acdemic Press, Inc.: Sn Diego, [8] Hkemi, H. Elstic constnts in dilute poly(styrene)/nemtic liquid crystl solutions - effects of concentrtion nd moleculr weight. Polymer 1999, 40, [9] Hori, H.; Urkw, O.; Adchi, K. Dielectric Relxtion in Phse-Segregted Mixtures of Polystyrene nd Liquid Crystl 5CB. Mcromolecules 2004, 37, [10] Benmoun, F.; Doudi, A.; Roussel, F.; Buisine, J.-M.; Coqueret, X.; Mschke, U. Equilirium Phse Digrm of Polystyrene nd 8CB. J. Polym. Sci., Prt B: Polym. Phys. 1999, 37, [11] Gogius, N.; Mschke, U.; Benmoun, F.; Ewen, B.; Coqueret, X.; Benmoun, M. Phse Digrms of Poly(dimethylsiloxne) nd 5CB Blends. J. Polym. Sci., Prt B: Polym. Phys. 2001, 39, [12] Nod, I.; Kto, N.; Kitno, T.; Ngsw, M. Thermodynmics of Modertely Concentrted Solutions of Liner Polymers. Mcromolecules 1981, 14, [13] Brndup, J.; Immergut, E. H.; Grulke, E. A., eds. Polymer Hndook. 4 th ed. 1999, Wiley: New York.

WILL A POLYMER DISSOLVE IN A GIVEN SOLVENT? WILL A PARTICULAR POLYMER MIX WITH ANOTHER POLYMER?

WILL A POLYMER DISSOLVE IN A GIVEN SOLVENT? WILL A PARTICULAR POLYMER MIX WITH ANOTHER POLYMER? MIXING WILL POLYMER DISSOLVE IN GIVEN SOLVENT? WILL PRTICULR POLYMER MIX WITH NOTHER POLYMER? POLYMER SOLUTIONS ND LENDS MISCILE IMMISCILE lends Single phase Phase separated Solvent (or polymer) molecules